This invention relates to a resin composition which is useful for optical purposes and is a blend of two kinds of polymers different in refractive index. One of the two polymers is an acrylic ester polymer and the other a copolymer of fluorine-containing compounds.
Research and development work on optical fibers and related optical devices and materials has been extended and accelerated with the rapid increase in practical applications of optical communication systems. Most of optical fibers already developed to the industrial production stage are formed of quartz glass and/or inorganic multicomponent glass, and some plastics optical fibers also have been developed to a practicable stage.
Advantages of plastics optical fibers reside in good flexibility and ease of fabrication and processing, though these optical fibers are inferior to inorganic glass optical fibers in respect of transmission loss. Accordingly large demand for plastics optical fibers is expected in the field of short-distance optical communications. In a practical sense conventional plastics optical fibers are limited to the step-index type fibers, which consist of a plastics core that exhibits a relatively high index of refraction and a plastics cladding that exhibits a lower index of refraction so that light propagates by repeating reflection at the core-cladding interface. Usually the core material is an acrylic resin represented by poly(methyl methacrylate), and the cladding material is selected from fluoro-polymers that are inherently low in refractivity by the effect of the atomic refraction of fluorine. Examples of useful fluoro-polymers are polyfluoroethylenes and copolymers of vinylidene fluoride with a fluoroethylene.
Researches have been conducted also on plastics optical materials of the graded-index type in which the refractivity is so distributed as to decrease gradually from the central region toward the periphery. Optical materials of this type are attracting increasing interest for use in optical communication cables and also in special lenses. Along with development of new optical devices and materials, need is felt for adhesives and coating materials which exhibit controlled indices of refraction and are useful for optical purposes.
As to means to produce plastics optical materials of the graded-index type or to desirably control the refractive indices of plastics materials, trials have been given to graft copolymerization, photocopolymerization, modification of known polymers by some chemical reaction, and blending of polymers different in refractive index. Among these techniques, blending of a polymer having a relatively high refractive index with another polymer lower in refractivity is deemed simple, easy and widely applicable. However, this method has rarely provided practicable optical plastics. The main reason for the unsuccess is that two kinds of polymers greatly different in refractivity are in most cases poor in mutual solubility, so that a blend of such polymers becomes low in mechanical strength and also in transparency when molded or extruded into a desired shape. Nevertheless, Japanese patent application primary publication No. 59-41348 (1984) shows that a blend of 5-50 wt % of poly(methyl methacrylate), or a copolymer of methyl acrylate and another methacrylate or an acrylate, with 95-50 wt % of a copolymer of vinylidene fluoride and tetrafluoroethylene is a blend system almost perfect in mutual solubility and is useful for optical purposes. According to Japanese patent application primary publication No. 59-62657 (1984), similarly good mutual solubility is exhibited also when a copolymer of vinylidene fluoride and trifluoroethylene was used in place of the copolymer in Japanese 59-41348.